PLSclear requires an ISBN or ISSN for all titles (unless the content is hosted on a website). If the title you are looking for does not have an ISBN/ISSN, please search for the title name in the first instance to see if it appears in the results.
Please note that if the title predates the ISBN/ISSN system (1970), you will need to refer your request to the publisher directly.
If the content is hosted online, you can search for the top-level URL of the website. Please see the help article 'I want to reuse website content' for more information.